Most of the steel on your new home relies on a thin metallic coating to stop it rusting. Pick the wrong one, or let the wrong material touch it, and you can have rust stains and leaks long before the roof should need any attention. Here's a plain-English rundown of what you'll see on site.
C8 - Metal Roof, Box Gutter and Flashing Inspection Checklist
$59.00 AUD
Last updated 5 Oct 2026 · Revision 1 · Free updates for life
Checklist Type: Minor
Form Factor: Fillable, print-ready PDF (A4, 20 pages)
When to use: Roofing stage (metal roof), before the linings go in
A 143-point checklist for metal roofs. Check roof sheets, fixings, gutters, downpipes, box gutters and flashings before the linings go in, when problems are easier to fix.
Galvanising: zinc on steel
Galvanising coats steel with zinc. The zinc keeps air and water off the steel and, if the coating gets scratched, the zinc corrodes first to protect the steel underneath. That's called sacrificial or galvanic protection.
There are two ways it's done:
- Hot-dip galvanising of fabricated items. The finished part is dipped in molten zinc. Think steel posts, lintels, stair stringers, brackets and bolts. This is covered by AS/NZS 4680.
- Continuous coating of sheet and coil. Flat steel runs through a zinc bath on a production line. Sheet steel is graded by coating mass under AS 1397, for example Z275 or Z450, which is the grams of coating per square metre counted over both sides.
On a house, galvanised steel turns up in lintels, posts, bracing straps, framing brackets and fasteners. It's a proven, affordable way to keep structural steel protected.
Zincalume: zinc and aluminium
Zincalume is BlueScope's brand of aluminium-zinc coated steel. The same coating is known overseas as Galvalume. The common grade for roofing is AZ150, about 150 grams per square metre over both sides.
The aluminium gives a tough barrier and the zinc gives the sacrificial protection at cut edges and scratches. In most environments it outlasts a straight galvanised coating of similar mass, which is why most new metal roofs and gutters in Australia are Zincalume or Colorbond rather than galvanised.
Colorbond: painted Zincalume
Colorbond is Zincalume steel with a factory-baked paint system on top. It's what most people mean when they say "a tin roof" these days. You get the metallic protection plus a coloured finish, and the paint adds another layer of protection.
What damages these coatings
This is the part that catches people out. Zincalume and Colorbond don't like:
- Wet concrete, mortar and lime. These are alkaline and attack the aluminium-zinc coating. Keep sheets, flashings and gutters clear of mortar splashes and don't store them on a slab where they'll sit in concrete slurry.
- Water running off copper, lead or CCA-treated timber. Runoff from these materials attacks the coating. Old copper pipes or a lead flashing upstream of new Zincalume is a problem.
- The wrong fasteners. Use fasteners rated for the roof material and your environment. Stainless or copper fittings in direct contact can cause trouble too.
- Swarf. Metal filings left on a roof after cutting and drilling rust within days and leave orange stains that can damage the paint. Your roofer should sweep the roof as they go.
Coastal sites
Salt is hard on all metal coatings, so distance from the sea matters. BlueScope's guidance is roughly:
- More than about 200 m from breaking surf: standard Colorbond is generally fine.
- About 100 to 200 m from breaking surf: Colorbond Ultra.
- Within about 100 m: stainless steel products.
Always check the manufacturer's current environment guide for your exact site, and make sure fasteners and flashings are rated for the same environment. Unwashed areas like eaves and under verandahs corrode faster because the rain never rinses the salt off.
Passivated zinc: small parts
Passivation is a light chemical treatment applied over zinc, mostly on small parts like screws, nails, nuts, washers and brackets. It slows early white rust and staining, especially while parts are in storage. It's not a long-life roofing or structural coating, so don't treat it as one. For anything exposed to weather, check the fastener's corrosion class suits your site.
What to check on your build
- The roof, gutters and flashings are the product and grade on your plans or contract.
- Coastal sites have the right product for the distance to the surf.
- No mortar, concrete or swarf is left on metal surfaces.
- Lintels and exposed steel are galvanised, and any cut or welded areas have been touched up with a zinc-rich paint.
Our C8 checklist walks you through the roof, box gutter and flashing inspection step by step.
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